The persistence of the tobacco smoking habit can be attributed in large part to the addictive actions of nicotine. Similar to all major drugs of abuse, nicotine excites brain reward systems. This stimulatory action of nicotine on reward systems underlies, or at least contributes to, it's positive reinforcing effects that maintain nicotine intake. In contrast, withdrawal from nicotine after chronic exposure decreases the activity of brain reward systems. Avoidance and alleviation of this withdrawal-associated reward deficit may also contribute to the persistence of the tobacco habit. An understanding of the mechanisms by which nicotine stimulates brain reward systems, and generates reward deficits during withdrawal, may lead to development of novel therapies for the treatment of habitual tobacco smoking. The overall hypothesis guiding this proposal is that nicotine acts at specific subtypes of nicotinic acetylcholine receptors (nAChRs), comprised of discrete nAChR subunits, to stimulate reward systems and generate reward deficits during withdrawal. Specific Aim I of this proposal will identify the nAChR subtypes that regulate nicotine consumption in mice. This goal will be achieved by identifying mice with targeted deletion of specific nAChR subunits (nAChR-/- mice) that demonstrate attenuated intravenous nicotine self- administration (SA) compared with wildtype (WT) littermates. Specific Aim II will identify nAChR subunits that regulate the stimulatory effects of nicotine on brain reward systems. This goal will be achieved by identifying nAChR-/- mice with attenuated nicotine-induced lowering of intracranial self-stimulation (ICSS) thresholds compared with WT mice. Convergent data from nicotine SA and ICSS experiments will provide compelling evidence for a role of specific nAChR subunits in nicotine reinforcement. Specific Aim III will identify nAChR subunits at which chronically administered nicotine acts to induce nicotine dependence and the expression of reward deficits during withdrawal. This goal will be achieved by identifying nAChR-/- mice with attenuated elevations of ICSS thresholds compared with WT mice during spontaneous withdrawal from chronically administered nicotine. These studies promise to yield significant new insights into the neurobiological mechanisms of nicotine addiction, with relevance for the treatment of the tobacco habit in human smokers.